1. Technical Field of the Invention
This invention relates to cellular telecommunication systems and, more particularly, to a system and method of locating a mobile terminal operating within the service area of a cellular telecommunication system.
2. Description of Related Art
In modern cellular telecommunication systems, the service area is divided into cells, each of which is served by one base station. As mobile terminals move about the service area of the system, they are handed off from one cell to another so that there is no lapse in service. In times of emergency, such location information would be extremely useful to police or emergency crews responding to the situation.
A major problem with existing solutions for locating mobile terminals is the high cost and complexity required for their implementation into the existing cellular network infrastructure. Some solutions require extensive modifications to base stations or other components of the cellular telecommunication system. Others require extensive modifications to mobile terminals and are inadequate due to the large number of unmodified mobile terminals already in service.
Although there are no known prior art teachings of a solution to the aforementioned deficiencies and shortcomings such as the solution disclosed herein, a number of prior art references exist that discuss subject matter that bears some relation to matters discussed herein. Such prior art references are U.S. Pat. Nos. 5,293,645 to Sood; 5,293,642 to Lo; and 5,208,756 to Song. Each of these references is discussed briefly below.
U.S. Pat. No. 5,293,645 to Sood (Sood) discloses a system and method of locating a movable radio terminal within a cellular telephone network. Sood requires that a plurality of base stations transmit synchronized timing reference signals. A receiver in the network receives a transmission from the radio terminal to be located that includes information compiled at the radio terminal indicating relative propagation delays in receipt of the timing reference signals from at least three base stations. A processor coupled with the receiver processes the transmission to convert the propagation delays into a geographic location of the radio terminal.
There are several disadvantages of the locating method disclosed in Sood. First, existing networks must be modified to enable all of the base stations to simultaneously transmit synchronized timing reference signals. Second, mobile radio terminals must be modified to compile information indicating relative propagation delays in the receipt of the timing reference signals from at least three base stations. Third, mobile terminals must be modified to transmit this compiled information. Fourth, a receiver must be modified or added to the network to receive the transmissions from mobile terminals indicating relative propagation delays. Fifth, a processor must be added to the network to convert the propagation delays into a geographic location of the radio terminal. Overall, Sood provides a very expensive and logistically complex method of locating mobile terminals.
U.S. Pat. No. 5,293,642 to Lo (Lo) discloses a method of estimating the location of a mobile station in a cellular communication system. The mobile station measures radio propagation parameters between the mobile station and each base station within propagation range of the mobile station. The measured parameters may include signal strength and radio propagation delay, and are used with base station antenna radiation pattern data to derive information concerning radio path attenuation. These parameters are then used to define a location probability density function for each individual base station. A joint probability density function is then constructed by combining the individual functions from each base station. The joint probability density function is then used to statistically estimate the probable location of the mobile station.
Like Sood, the solution disclosed in Lo suffers from several disadvantages. First, the cellular communication system and mobile stations must be modified to detect and compute the various radio propagation parameters for each base station. Lo does not disclose how parameters such as radio path attenuation are to be determined, but states only that they may be determined by means of radio signal strength measurement techniques using a vehicle mounted test transmitter, or by theoretical models of propagation, or by a combination of the two. It would be a very expensive and time-consuming operation to determine such radio propagation parameters for the coverage area of every base station in the communication system. Second, this large amount of data must be stored and readily recalled for use in rapidly calculating probability density functions. Third, additional processors must be added to the communication system for performing these functions. Thus, Lo also provides a very expensive and logistically complex method of locating mobile terminals.
U.S. Pat. No. 5,208,756 to Song (Song) discloses a vehicle locating and navigating system using a cellular telephone network. In Song, a mobile telephone device measures relative signal strengths of control signals received from several base stations. The device then calculates the distance between the vehicle and each base station as a function of the transmitted power, received power, and signal attenuation factors of the signals transmitted by each base station. The device then uses trilateralization or arculation to determine the location of the vehicle as a function of the calculated distances and the known locations of the base stations.
Song suffers from the disadvantage that major modifications must be made to existing mobile telephones to implement the Song method. First, a memory device must be added to store information on each base station such as geographic location and transmitter power. Song does not disclose how data such as signal attenuation factors for each base station and location in the coverage area are to be determined, nor does Song disclose how such a large amount of data is to be stored in the mobile telephone device. Second, this information must be updated if additional base stations are added, transmitter power changes, or any other signal attenuation factors change. Third, a processor must be added to calculate distances from each base station and perform the trilateralization or arculation required to determine the geographic location of the mobile telephone. Thus, Song also provides a very expensive and logistically complex method of locating mobile terminals.
Review of each of the foregoing references reveals no disclosure or suggestion of a system or method such as that described and claimed herein.
It would be a distinct advantage to have a system and method of locating mobile terminals in a cellular telecommunication system that does not require base stations to transmit synchronized timing reference signals and does not require extensive modifications to the telecommunication system infrastructure or to the thousands of existing mobile terminals. The present invention provides such a system and method.